This invention relates to rare earth borates, especially yttrium-europium borates and yttrium-gadolinium-europium borates, useful as phosphor raw material, and a method for preparing the same.
Yttrium-gadolinium-europium borate phosphor finds use in displays as a red phosphor. Especially in the recent years, engineers in the plasma display art are interested in it as a phosphor capable of light emission at a high efficiency in response to exciting radiation of 147 nm from a xenon plasma.
In general, the yttrium-gadolinium-europium borate phosphor is prepared by mixing yttrium-gadolinium-europium oxide co-precipitates with boric acid in powder form and subjecting the mixture to reaction at elevated temperature.
For such display panels as plasma display panels (PDP), to increase their brightness is an important task in improving display performance.
The brightness of panels can be increased, for example, by increasing the brightness of phosphor itself. It is believed that the brightness of panels is largely affected by the coating property of phosphor to cells. With respect to the coating property of phosphor, those phosphors which can be applied to plasma display cells in a uniform, least irregular state are regarded preferable. With respect to the shape of phosphor, particles of small size, equal diameter and identical shape are regarded preferable because uniform coating property is improved.
The particle size and shape of phosphor, especially the particle size of phosphor, depend on the particle size of a raw material or rare earth oxide. In general, using a raw material having a less variation of particle diameter or a sharper particle size distribution, a phosphor having a sharper particle size distribution is obtained. Recently, the raw material powder is thus required to have a sharper particle size distribution.
However, a microscopic observation of conventional raw material oxide revealed that even a raw material powder having a sharper particle size distribution contained particles of differing size. Such a raw material powder was regarded to have a sharp particle size distribution for the mere reason that the difference in particle size was relatively small or particles had somewhat similar shapes. The phosphor prepared from such a raw material powder contains particles of differing size.
An object of the invention is to provide a rare earth borate in the form of spherical, uniform diameter particles useful as a phosphor raw material, and a method for preparing the same.
The invention provides, in one embodiment, a rare earth borate in the form of substantially spherical, uniform diameter particles having an average particle diameter of 0.5 xcexcm to 2 xcexcm; in another embodiment, a yttrium-europium borate in the form of substantially spherical, uniform diameter particles having an average particle diameter of 0.5 xcexcm to 2 xcexcm; and in a further embodiment, a yttrium-gadolinium-europium borate in the form of substantially spherical, uniform diameter particles having an average particle diameter of 0.5 xcexcm to 2 xcexcm.
According to the method of the invention, a rare earth borate in the form of substantially spherical, uniform diameter particles having an average particle diameter of 0.5 xcexcm to 2 xcexcm is prepared by mixing yttrium-gadolinium-europium oxide co-precipitates in the form of substantially spherical, uniform diameter particles having an average particle diameter of 0.5 xcexcm to 2 xcexcm with boric acid or boron oxide in a molar ratio of 1/1 to 1/3, and heating the mixture at 600xc2x0 C. to 1,100xc2x0 C. for reaction.
The inventors previously proposed in U.S. Ser. No. 09/897,463, now pending (Japanese Patent Application No. 2000-203549) a substantially spherical rare earth oxide having a uniform particle diameter and shape and a method for preparing the same. The inventors have found that by mixing boric acid or boron oxide with the substantially spherical rare earth oxide of uniform particle diameter and uniform shape having an average particle diameter of 0.5 to 2 xcexcm obtained by the method of the preceding application and heating the mixture for reaction, there is obtained a rare earth borate of uniform particle diameter and uniform shape, which is well suited as a phosphor raw material.